As is known, diffraction and dispersion are phenomena that limit the applications of beams and pulses of electromagnetic and acoustic waves.
Diffraction is present whenever a wave is propagated in a medium, producing a continuous spatial widening. Said effect constitutes a limiting factor in remote-sensing applications and whenever it is necessary to generate a pulse that will maintain its own transverse localization, such as, for example, in free-space communications, in electromagnetic “tweezers”, etc.
The dispersion acts on pulses that propagate in a material, and mainly generates a temporal widening of the pulses on account, as is known, of the different phase velocity for each spectral component of each pulse (due to the variation of the index of refraction of the medium as a function of frequency). Consequently, a pulsed signal may undergo degradation due to a temporal widening of its spectrum, which is undesirable. The dispersion is hence a further limiting factor when there is the need for a pulse to maintain its own spectral characteristics, in particular its width over time, such as, for example, in communications systems.
It is thus important to develop techniques that will be able to reduce these undesirable phenomena.
The so-called “localized waves” (LW), which are also known as non-diffractive waves, have the property of withstanding diffraction for a long distance in free space, propagating with only slight dispersion. Today, concept of localized waves is well consolidated both from a theoretical standpoint and from an experimental standpoint, and localized waves are applied successfully in innovative applications both in a medium that in a vacuum, featuring a good resistance to dispersion.
Systems that use localized waves can find valid application in investigation at a distance for identifying buried objects, such as, for example, in the sectors of archaeology, minesweeping, long-distance wireless power transmissions, anticrash systems, electromagnetic propulsion systems, molecular-excitation systems for conservation of quantum angular momentum, for safe medium-distance communications, etc.
The most important and peculiar part of a localized-wave system is constituted by the radiating structure (antenna). Radiating structures are typically obtained by means of one of the following configurations: shields with circular slits impinged upon by plane waves, recollimated by means of lenses; arrays of appropriately phased acoustic emitters (transducers); electromagnetic radiators made with multimodal waveguide; “axicons” (optical components with at least one conical surface); and holographic elements.
So far, considerable attention has been dedicated to application of localized waves to systems operating in the optical and acoustic domains. In the field of microwaves there has been an attempt to imitate optical configurations, and the technological developments have been slowed down by the need to use radiating structures that are dimensionally very large (given that the overall dimensions of said radiating structures are determined by the wavelength of the electromagnetic signal applied to the radiating structure).
These radiating structures are, consequently, costly and cumbersome to produce.